WO 09/049,000 has already disclosed a method of the type specified above, that uses radiation-curable coating compositions which, in addition to >10% to <40% by weight of nanoparticles (B), based on the total weight of the film-forming constituents (A) and (C), comprise 10% to 60% by weight of at least one binder (A) and 40% to 90% by weight of at least one reactive diluent (C), the sum of the weight fractions of the film-forming constituents (A) and (C) being in each case 100% by weight. The resulting coatings feature good abrasion resistance and very good optical properties, i.e., a low haze value for the unexposed coating of less than 1. These qualities are assured by WO 09/049,000 by using urethane (meth)acrylates having two double bonds per molecule as binder (A) and high-functionality reactive diluents having preferably 3 to 6 double bonds per molecule as reactive diluent (C). If, in contrast, hexafunctional urethane (meth)acrylates and difunctional reactive diluents, such as hexanediol diacrylate, or only trifunctional and pentafunctional reactive diluents, are used, then the very low haze values required are not attained.
Moreover, EP-B-1 704 189 discloses radiation-curable coating compositions for coating plastics substrates, more particularly polycarbonate substrates, said compositions, in addition to nanoparticles (B), comprising at least two different polyfunctional (meth)acrylate-functional compounds, more particularly a mixture of a hexafunctional urethane acrylate and at least one compound selected from the group consisting of butanediol diacrylate, trimethylolpropane triacrylate, and pentaerythritolyl triacrylate. This combination of two different polyfunctional (meth)acrylate-functional compounds enhances the abrasion resistance of the resulting cured coating by comparison with cured coatings based only in each case on one polyfunctional (meth)acrylate-functional compound. That specification, however, does not give any information on the optical properties of the resulting coatings or on the effect of the film-forming components on the optical properties.
Also known, furthermore, from the conference report “RadTech 2000” of Apr. 9-12, 2000, Baltimore, pages 822-831, radiation-curable coating compositions for coating plastics substrates, more particularly polycarbonate substrates, said compositions, in addition to nanoparticles (B), comprising at least two different polyfunctional (meth)acrylate-functional compounds, more particularly a mixture of a hexafunctional urethane acrylate and at least one compound selected from the group consisting of hexanediol diacrylate, tripropylene glycol diacrylate, and ethoxylated trimethylolpropane triacrylate.
Disclosed, furthermore, by U.S. Pat. No. 6,420,451 are radiation-curable coating compositions, intended more particularly for the coating of spectacle lenses, that comprise (a) 20% to 80% of a first aliphatic urethane acrylate, more particularly a urethane acrylate having on average 2 double bonds per molecule, (b) 5% to 50% of a compound having one acrylate group per molecule, (c) (i) 2% to 30% of a second aliphatic urethane acrylate, more particularly a urethane acrylate having on average 6 double bonds per molecule, or (ii) 2% to 25% of a polyfunctional acrylate compound, or (iii) a combination of (i) and (ii), and (d) 1% to 30% of nanoparticles.
EP-B-668 330 describes radiation-curable coating compositions intended for the coating of polycarbonate substrates and comprising 20% to 75% by weight of at least one substantially hydroxyl-free and isocyanate-group-free aliphatic urethane acrylate (A) based on low-viscosity polyisocyanates containing isocyanurate groups, 5% to 80% by weight of a low-viscosity acrylic ester component (C), composed to an extent of at least 80% by weight of a bis-functional acrylic ester component and to an extent of up to 20% by weight of another acrylic ester component, and 0% to 80% by weight of a solvent or solvent mixture, the weight percentage figures for components (A), (C), and (LM) (solvent) being based in each case on the total weight of components (A), (C), and (LM). That specification, however, does not describe the addition of nanoparticles to the coating materials. Nor does it provide any information on the optical properties of the resulting coatings or on the effect of the film-forming components on the is optical properties.
Finally, U.S. Pat. No. 4,455,205 discloses radiation-curable coating compositions which comprise at least two different polyfunctional (meth)acrylate-functional compounds, more particularly a mixture of hexanediol diacrylate and trimethylolpropane triacrylate, and the hydrolysis product of silyl-functional acrylates and colloidal silicon dioxide. The use of urethane (meth)acrylates in the coating compositions, however, is not described.
The problem on which the present invention is based, then, was that of providing a method of coating plastics substrates, more particularly casings of electronic appliances, which endows the resulting coated substrates with a combination of very high scratch resistance with very high transparency and low gray haze (haze values for the unexposed coating of less than 1, preferably less than 0.8, in each case as determined using the BYK-Gardner Haze-gard plus C4725 instrument).
Furthermore, the coating compositions, following application to the plastics substrates, more particularly to polymethyl methacrylates, polycarbonates and/or blends of polycarbonate and other plastics, ought not to lead to any incipient swelling of the substrates.
Moreover, the coating compositions ought to result in cured coatings having very good adhesion (more particularly as determined using tape pull-off in accordance with ASTM D 3359 and ISO 2409) on plastics, more particularly on polymethyl methacrylates, polycarbonates and/or blends of polycarbonate and other plastics.
Lastly, the cured coatings ought to exhibit a high level of resistance on impact deformation (impact test) as determined in a method based on the standard DIN EN ISO 6272-1 DE.